Subsurface contamination of soil, groundwater, and perched water has become a concern because such contamination has been associated with a number of detrimental health risks. Subsurface contamination that includes perchlorate salts is a particular concern because the salts are soluble in water and capable of being transported long distances in underground aquifers. The U.S. Environmental Protection Agency has proposed a maximum contaminant level (MCL) for perchlorates of about 1 part per billion in water.
Perchlorate present in soil can leach into groundwater, and infiltrating storm water can increase the size of a contamination area. Several perchlorate plumes in California are believed to measure miles in length. Hence, perchlorate contamination of soil overlying groundwater is of particular concern. Continuing contamination of groundwater by perchlorate sources in soil can result in the need to treat groundwater for extended time periods at a much greater cost. It is believed that treatment of perchlorate sources in soil will result in faster cleanup of groundwater and decrease costs.
Various approaches for treating contaminated ground water have been developed. Ex situ treatment methods usually involve removing some portion of contaminated ground water, treating it, and reintroducing the treated water to the subsurface. Examples of ex situ treatment processes include ion exchange, anaerobic biological treatment, reverse osmosis, and tailored activated carbon adsorption.
A more limited range of in situ treatment methods have been developed such as, for example, anaerobic biological treatment. In situ anaerobic biological treatment has generally been conducted two different ways. The first method involves injecting a liquid electron donor into the groundwater. An electron donor is a chemical capable of donating electrons to bacteria to assist the bacteria in biologically reducing contaminants to innocuous byproducts. One problem with this approach is that dispersion of injected liquid electron donors is limited and requires the use of closely spaced injection points. A second method involves extracting groundwater, mixing the extracted groundwater with liquid electron donors, and then re-injecting the groundwater with the liquid electron donor. One problem with this second approach is that pumping costs for groundwater can be high and the wells and equipment can be biofouled by the injected liquid electron donor.
A variety of techniques have also been developed for treating contamination of shallow soil. In one method, the contaminated shallow soil is simply excavated and disposed in a landfill. It has been discovered that some microbial species (e.g., some bacterial species) are able to utilize some contaminants in various metabolic processes (e.g., anaerobic processes). These processes can be stimulated by introducing an electron-donating material into the subsurface. Some in situ and ex situ anaerobic biological treatment methods have also been developed, however, these are limited to shallow soils and use liquid or solid electron donors that are difficult to deliver in situ.
Previous techniques for treating contamination of deep soil are extremely limited and have not been successful. One example includes the percolation of liquid electron donors to deep soil, but these methods have been limited by channeling of the liquid electron donors through the soil and incomplete contact between electron donor, the contaminant, and bacteria. Other examples of remediation techniques for deep soil include in situ thermal treatment, aerobic biological treatment or bioventing, and soil vapor extraction; however, these techniques are not very effective at treating perchlorate contamination. Thermal treatment of deep soil perchlorate contamination is not very effective as perchlorates are stable at typical thermal treatment temperatures. Aerobic biological treatment or aerobic bioventing is not an effective approach as perchlorates are more effectively degraded under anaerobic conditions. Also, perchlorates are usually not volatile, so soil vapor extraction is not applicable to perchlorates.
In addition to perchlorates, nitrate subsurface contamination is also a concern. While nitrates are not of as much concern as perchlorates, nitrate contamination is more widespread because it is commonly used to fertilize crops and grass. Other contaminants of concern are halogenated volatile organic compounds.
Previous work by the U.S. Engineer Research and Development Center (ERDC) demonstrated that the gaseous electron donor isobutyl acetate promoted biodegradation of trinitrobenzene also called TNB (Rainwater, et al., Design, Construction, and Operation of a Field Demonstration for In Situ Biodegradation of Vadose Zone Soils Contaminated With High Explosives, ENVIRONMENTAL LABORATORY ERDC/EL TR-01-28 (September 2001)). Rainwater, et al. tested other gaseous electron donors including ethanol, acetone, and acetic acid, but those donors did not promote biodegradation of RDX and TNB compared to the nitrogen-only control. Only nitrogen gas alone was shown to promoted biodegradation of RDX and TNB. The data presented by Rainwater, et al. indicated that gaseous electron donor addition is poor technology for remediation of RDX and TNB and is limited to the exclusive use of isobutyl acetate for the remediation of TNB.
A need exists, therefore, for better methods of treating subsurface contaminants (e.g., perchlorates, nitrates, and halogenated volatile organic compounds) in situ.